1. Field of the Invention
The present invention relates to a thin film forming device for forming a thin film on a substrate in a vacuum, particularly to that capable of observing and analyzing the state or process of the thin film growing on the substrate.
2. Description of the Related Art:
A representative thin film forming device is a molecular beam epitaxy device (referred to as an MBE device hereinafter). The MBE device comprises a molecular beam source such as a Knudsen cell (referred to as a K cell hereinafter) etc. disposed toward a substrate mounted on a substrate holder in a vacuum chamber which is kept at a high vacuum, the MBE device emitting therefrom molecules of material of the thin film toward a film-forming surface of the substrate to grow the thin film thereon. In case of the K cell which emits molecules by electric heating, a heater is covered by a shrouding which is cooled by liquid nitrogen etc.
In case an oxide film or a compound semiconductor film such as a high-temperature superconducting thin film etc. is formed in such an MBE device, it is performed under irradiation of a beam of electrically neutral and chemically active free radicals generated in plasma. In this case, a plasma beam source is disposed in the vacuum chamber toward the film-forming surface of the substrate adding to the molecular beam source described above.
Moreover it makes use of reflection high-energy electron diffraction (RHEED), wherein an electronic beam accelerated to 10.about.50 KeV is incident to the film-forming surface at a small angle so as to be diffracted thereon so that a diffraction pattern is projected on a fluorescent screen for observing the crystal lattice of the film, In this case, an electron gun is disposed in the vacuum chamber toward the film-forming surface of the substrate at a small angle relative thereto and the fluorescent screen is put over an observation window which is disposed at a side of the substrate opposite to the electron gun.
In the thin film forming device as described above, a film formed on the film-forming surface of the substrate is taken out from the vacuum chamber after completion of forming the film and the composition thereof is analyzed by photo-luminescence etc. This method, however, is ineffective in knowing the composition of the film while the same is being formed. On the other hand, the RHEED is effective in knowing a crystal lattice in the film while the same is being formed, but ineffective in knowing the composition of mix crystal such as impurity densities etc.
Furthermore, in case the film is formed on the film-forming surface of the substrate under irradiation of the electrically neutral and chemically active free-radical beam, there is no other way to observe the plasma radiation of the plasma beam source than through the observation window provided on the vacuum chamber. Such a method of observation, however, is incapable of directly observing the plasma radiation. Accordingly, there has been no other way to know the state of plasma radiation than surmising from indirect data such as the voltage, current, etc. of a power supply to the plasma beam source.
Still furthermore, there has also been no other way to know the remaining amount etc. of a material of the molecular beam source such as the K cell etc. during the formation of the film than indirectly surmising from the speed of formation of the film and the like. Still furthermore in case of the K cell, a shrouding provided around a crucible for accommodating the material of the film therein is cooled to a low temperature so that the material is sometimes piled at the irradiation hole of molecules to close it. Such a situation cannot be observed through the observation window provided on the vacuum chamber.